regression: d3d11: black screen instead of visualization

A regression was introduced among one of the commits backported to 3.0 on Wednesday 17th.

Steps to reproduce:

Direct3D11 video output
Enable a visualization (spectrometer, scope, spectrum, vu-meter, goom) => Black screen

I can reproduce in 4.0 too.

VLC: 3.0.5-20181018-0533-win64\ OS: Windows 10 x64 1803

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